We had the same problem with the flatter tube submerging
Finally found one that's about as steady and unflippable (if there is such a word) as one can get. It is called "Big Bertha". We got ours at a local store called Fleet and Farm, which is nothing more than a MAN'S store. Just google Big Bertha and you should be able to get to any store that handles them. I've seen others, maybe even Overtons. We only paid $100.00 for the thing and it is absolutely safe.